1. Field of the Invention
The present invention relates to a method of generating synchronizing signals, and more particularly to a method of generating synchronizing signals for determining a point from which to start effective scanning on each scanning line in a light beam scanning process.
2. Description of the Prior Art
Light beam scanning has heretofore been employed to scan a recording medium with a light beam for reading out any of various forms of information, such as image information, recorded on the recording medium, or recording such information on the recording medium.
In the light beam scanning, a light beam such as a laser beam is deflected by a light deflector such as a rotating polygonal mirror to scan a recording medium (main scanning), while at the same time the recording medium is moved (subscanning) in a direction substantially normal to the direction in which the recording medium is scanned by the light beam. When information is to be recorded on the recording medium, the light beam is modulated with an information signal by an acousto-optic modulator, and the recording medium is scanned by the modulated light beam. For reading out information from the recording medium, the light beam is scanned over the recording medium, and secondary light, such as transmitted light or reflected light, which has been generated from the recording medium by the scanning, is detected to read out the recorded information.
One method of reading out recorded information with light beam scanning is disclosed in U.S. Pat. No. 4,410,799, for example. According to this method, a stimulable phosphor sheet with radiation image information recorded thereon is scanned with stimulating light such as a laser beam to emit secondary light, which is photoelectrically read out by a photoelectric read-out means such as a photomultiplier.
In the disclosed light beam scanning process, it is generally necessary to align points from which to start effective scanning on respective main scanning lines that are formed on the recording medium by a light deflector. For example, the points from which to start effective scanning are aligned with each other in the subscanning direction to eliminate any positional displacement thereof in the main scanning direction. If these effective scanning starting points were positionally displaced from each other in the main scanning direction, a phenomenon called "jitter" is produced, distorting or otherwise adversely affecting the recorded or read-out image information.
To prevent the scanning starting points from being positionally displaced, there has been proposed a method of generating a synchronizing signal when a light beam passes through a predetermined reference position on each scanning line. Effective scanning on each scanning line is started on the basis of the generated synchronizing signal, i.e. simultaneously with the synchronizing signal or upon elapse of a given period of time from the generation of the synchronizing signal. Upon the start of the effective scanning, the scanning light beam begins to be modulated by the information signal for recording information, or the secondary light emitted from the recording medium by the scanning light beam begins to be read out for reproducing the recorded information.
FIGS. 8 and 9 of the accompanying drawings illustrate a conventional arrangement for generating a synchronizing signal. A light beam 2 emitted from a light source 1 passes through a collimator lens 3 and a cylindrical lens 4 and falls on a rotating polygonal mirror 5. The light beam 2 is then reflected by one of reflecting surfaces 5a of the mirror 5 and travels through a toric lens 6 and an f.theta. lens 7, falling on a recording medium 8. A synchronizing signal generator 9 comprising a slit plate 9a and a photoelectric detector 9b is disposed laterally of the recording medium 8. In each scanning cycle, the light beam 2 passes through a slit 9c of the slit plate 9a and is detected by the photoelectric detector 9b, which then produces an output signal serving as a synchronizing signal. The output signal (synchronizing signal) from the photoelectric detector 9b has a waveform as shown in FIG. 10. A time t1 at which the synchronizing signal attains a given level is employed as a reference time. The position which is reached by the light beam 2 upon elapse of a given period of time from the reference time t1 is determined as a point from which to start effective scanning.
During operation, the polygonal mirror 5 rotates about its own axis in the direction of the arrow A to deflect the light beam 2 in the direction of the arrow B, thereby scanning the recording medium 8 in the direction of the arrow C (main scanning). In one scanning cycle, the recording medium 8 is scanned by the light beam 2 which is deflected in one sweep in the direction of the arrow B by angular movement of one reflecting surface 5a.
However, the prior method of determining effective scanning starting points with respect to the reference time has proven unsatisfactory. It has been found that the reference time for the synchronizing signal to reach the given level should not be subject to fluctuation, or any fluctuation of the reference time should be extremely small.
The synchronizing signal generated by the conventional method has a rise time .DELTA.t1 or a maximum signal level which varies due to reflecting irregularities of the reflecting surface of the rotating polygonal mirror, variations in the speed of rotation of the rotating polygonal mirror, and blocking of the light beam by the lens system. Such variations in the rising transition period of the synchronizing signal cause the time t1 to fluctuate within the rise time .DELTA.t1. Since the synchronizing signal generated by the prior method has a large rising transition period, i.e. the rise time .DELTA.t1 is long, the time t1 tends to fluctuate to a large extent. Therefore, the conventional synchronizing signal has not been accurate enough to bring the effective scanning starting points into mutual alignment.